1. Field of the Invention
The present invention relates to an antiskid brake controller for preventing wheels from tending to be locked when a brake is applied, on a road surface on which a four-wheel-drive vehicle travels, and more specifically, to an antiskid brake controller capable of preventing a cascade (an unstable state) when a braking pressure is reduced while all the wheels are continuously driven.
2. Description of the Related Art
Conventionally, there have been well known antiskid brake controllers called ABS which avoid a locked state by reducing a braking pressure when the locking tendencies of wheels are detected, based on wheel speeds detected by wheel speed sensors when a brake is applied, slip amounts presumingly calculated from the wheel speeds or the like.
However, in four-wheel-drive vehicles, when a braking pressure is imposed on respective wheels to which engine torque is transmitted under respective conditions, a locking tendency or a slip is liable to be caused by an effect of the wheels different from a normal wheel lock, from which an unstable state called a cascade results.
To prevent the above problem, there has been proposed an antiskid brake controller which can switch a driving state from a four-wheel-drive-state to a two-wheel drive-state when ABS control is executed as shown in, for example, Japanese Unexamined Patent Publication No. 7-205790.
In this case, the reduction of the braking pressure applied to a wheel whose wheel speed is reduced by the occurrence of a slip is increased to thereby eliminate the locking tendency of the wheel.
FIG. 10 is a block diagram showing the schematic arrangement of an ordinary antiskid brake controller, FIG. 11 is a view specifically showing the arrangement of a hydraulic passage in the vicinity of actuators in FIG. 10 and FIG. 12 is a view showing the arrangement of the actuators in FIG. 11 in more detail paying attention to a wheel.
In the respective drawings, all the four wheels 1a-1d, that is, all of the front wheels 1a, 1b and the rear wheels 1c, 1d of a four-wheel-drive vehicle act as driving wheels coupled with an engine (not shown).
Wheel speed sensors (wheel speed sensing means) 2a-2d, of an electromagnetic pickup system or the like, individually detect the rotational speeds of the respective wheels 1a-1d as wheel speed signals Va-Vd.
Braking units 7a-7d each composed of a wheel cylinder are individually disposed to the respective wheels 1a-1d and pressed thereagainst in accordance with braking pressures Pa-Pd supplied from the actuators 10a-10d.
A master cylinder 9 coupled with a brake pedal 8 creates a braking pressure (hydraulic pressures) in response to an amount of depression of the brake pedal 8 and supplies it to the actuators 10a-10d including electromagnetic solenoids through a hydraulic pipe.
The actuators 10a-10d adjust the braking pressure supplied from the master cylinder 9 in accordance with control signals Ca-Cd and CM (to be described later) and individually supply the thus adjusted braking pressures to brake units 7a-7d.
With this operation, the brake units 7a-7d generate braking forces to the respective wheels 1a-1d in response to the amount of depression of the brake pedal 8 as well as in accordance with the control signals Ca-Cd and CM.
In FIG. 10, an ECU (electronic control unit) 11 provided with the vehicle constitutes the main body of the antiskid brake controller and includes waveform shaping/amplifying circuits 20a-20d, a power supply circuit 22, a microcomputer 23, actuator driving circuits 24a-24d and a motor relay driving circuit 25.
The power supply circuit 22 supplies a constant voltage to the microcomputer 23 when an ignition switch 27 is turned ON.
The microcomputer 23 includes a CPU 23a, a RAM 23b and a ROM 23c.
The ECU 11 calculates wheel speeds Vwa-Vwd from the respective wheel speed signals Va-Vd as well as individually calculates wheel decelerations corresponding to the locking tendencies of the respective wheels 1a-1d based on the differential waveforms of the wheel speeds Vwa-Vwd.
Further, the ECU 11 calculates control amounts sent to the braking force adjustment means composed of the respective actuators 10a-10d and a motor 15 (motor relay 16), creates the control signals Ca-Cd and CM for preventing the locking tendencies and adjusts the braking pressures Pa-Pd to the respective wheels 1a-1d.
The respective actuator driving circuits 24a-24d individually output the control signals Ca-Cd to the electromagnetic solenoids of the respective actuators 10a-10d in response to a control command from the microcomputer 23.
The motor relay driving circuit 25 outputs the control signal CM to the motor relay 16 when the braking pressure is to be adjusted and turns ON the normally open contact of the motor relay 16 by energizing the coil 16b of the motor relay 16 to thereby drive the motor 15.
With this operation, the motor 15 which constitutes a braking pressure adjustment pump adjusts the braking pressure Pa-Pd in cooperation with the actuators 10a-10d.
In FIG. 11, a reservoir tank 14, which communicates with the motor 15, supplies and collects a hydraulic pressure to and from the respective actuators 10a-10d through a hydraulic passage communicating with the respective actuators 10a-10d.
FIG. 12 shows only one of the actuators (for example, the actuator 10a) in FIG. 11, the actuator 10a including a braking pressure maintaining solenoid valve 12 and a braking pressure reducing solenoid valve 13. The other not shown actuators 10b-10d also have the same arrangement.
The pressure maintaining solenoid valve 12 is in the inlet hydraulic passage from the master cylinder 9 to the brake unit 7a and the pressure reducing solenoid valve 13 is connected to the outlet hydraulic passage from the brake unit 7a to the reservoir tank 14.
That is, the pressure reducing solenoid valve 13 is connected to the liquid pressure collecting passage from the reservoir tank 14 to the master cylinder 9, the passage going through the motor 15, for supplying and collecting the liquid pressure.
With this arrangement, the respective solenoid valves 12, 13 are energized or deenergized in response to the control signal Ca from the ECU 11 to thereby execute switching in order to maintain, increase, or reduce the braking pressure.
Ordinarily, the pressure maintaining solenoid valve 12 is opened and the pressure reducing solenoid valve 13 is closed when the antiskid brake controller is not in operation.
Next, ordinary ABS control operation will be described.
In FIG. 12, when the driver depresses the brake pedal 8, a pressure is supplied to the master cylinder 9 and a braking fluid fed from the master cylinder 9 flows into the brake unit 7a through the pressure maintaining solenoid valve 12 in the actuator 10a to thereby increase the braking pressure Pa.
When a wheel deceleration corresponding to a locked state is detected and the control signal Ca indicating pressure reduction is created by the ECU 11, the electromagnetic solenoids of the pressure maintaining solenoid valve 12 and the pressure reducing solenoid valve 13 are driven by being energized.
With this operation, the pressure holding solenoid valve 12 is closed to thereby shut off the hydraulic passage from the master cylinder 9 to the brake unit 7a and the pressure reducing solenoid valve 13 is opened to thereby connect the hydraulic passage from the brake unit 7a to the reservoir tank 14.
Therefore, the braking pressure Pa in the brake unit 7a flows into the reservoir tank 14 and is reduced.
At the same time, since the ECU 11 creates the control signal CM for operating the motor 15, the pressure of the braking fluid having flowed into the reservoir tank 14 is increased and the braking fluid having the increased pressure is returned to the main passage on the master cylinder 9 side to be used in the next brake control.
Thereafter, when the ECU 11 creates the control signal Ca for maintaining pressure and only the pressure maintaining solenoid valve 12 is energized (the passage is closed), since the other valves are deenergized, all the hydraulic passages are shut off and the braking pressure Pa to the wheel 1a is maintained.
When the ECU 11 creates the control signal Ca for increasing pressure and the pressure maintaining solenoid valve 12 and the pressure reducing solenoid valve 13 are deenergized, the hydraulic passage between the master cylinder 9 and the brake unit 7a is connected again.
With this operation, since the high pressure braking fluid having been returned to the main passage on the master cylinder 9 side flows into the brake unit 7a again together with the braking fluid discharged from the motor 15, the braking pressure Pa to the wheel 1a is increased.
As described above, although the braking pressures are conventionally controlled while avoiding the locking tendencies, since traveling conditions and road surface conditions are different in respective wheels, it is difficult to properly avoid the locking tendencies of all the wheels of four-wheel-drive vehicles.
In particular, there is a problem in the four-wheel-drive vehicles that since a braking pressure to a wheel is large, slips are caused in other wheels, and the braking pressure cannot be properly adjusted in such a case.
As described above, the conventional antiskid brake controllers reduce or increase a braking pressure by detecting a slip state in each of the wheels. However, since a wheel is coupled through the non-differential limit mechanism of the other wheel (or directly) in the four-wheel-drive vehicle so that power is transmitted to all the wheels, there is a problem that a locked state cannot be avoided by properly adjusting the braking pressures to all the wheels.
An object of the present invention made to solve the above problem is to provide an antiskid brake controller capable of avoiding a cascade (an unstable state) which is caused when a braking pressure is reduced in four-wheel-drive vehicles.